drm: Always warn if user-defined modes are not supported

Print a warning if a user-specifed display mode is not supported by
the display pipeline. Users specified the display mode on the kernel
command line with the use of the video= parameter. Setting an
unsupported mode will leave the console blank, so we should at least
let the user know why.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220511183125.14294-2-tzimmermann@suse.de